The main objective of the proposed research is to identify and analyze the causes of stalls in fertility declines in populations that have not reached the end of the demographic transition. Contrary to expectations based on conventional descriptions of the demographic transition, fertility has in many cases not continued declining at a steady pace after rapid reductions at the beginning of the fertility transitions. By the early 1980s a large proportion of countries in mid-transition had experienced stalling. The central assumption of the proposed research is that stalling is an unintended result of voluntary individual decisions regarding the quantum and timing of the fertility of couples. Adjustment to earlier mortality declines as well as an increase in age at marriage and birth interval duration are proposed as the main causes of stalling. To examine these hypotheses in detail, a quantitative analytic model that links period total fertility rates to trends in desired family size will be developed. This model is based on earlier work by Lee, which is expanded here by taking into account the effects of several confounding factors: variation in age of marriage, celibacy, marital disruption, sterility, sex preferences, infant and child mortality, unwanted fertility and trends in birth spacing. The model is then tested by applying it to 17 countries which have the necessary data from a World Fertility Survey. The validity of the results of these applications are compared with projections of stalling obtained by computer simulation. Once the model development has been completed the relationship between period fertility rates and desired family size and the stalling phenomenon will be examined with data from 15 more data sets from Demographic and health Surveys. An in- depth analysis of countries with multiple surveys is planned. In addition to providing an explanation for the stalling phenomenon, the model linking period fertility to desired family size will provide a theoretical framework that will bring measures of fertility preferences into the mainstream of quantitative demographic analysis. This should lead to improved projections of future trends in fertility in both developed and developing countries.